INSIGHTS on Centrifuges

Centrifuges work on the principle of sedimentation facilitated by an apparent angular force that draws components of a rotating sample away from the center of rotation. Although centrifugation theory is straightforward, its engineering literature is voluminous due to the number of industries and research operations that depend on the operation.

What’s important here is that centrifugation efficiency is proportional to the spinning radius and to the square of the angular velocity (radians per second, generally referred to as speed in revolutions per minute, rpm).

Thus, for a given rpm value a centrifuge with a 12-inch radius will be twice as efficient as one with a six-inch radius, and for a constant radius a device spinning at 1,000 rpm is four times as effective as one rotating at 500 rpm.

Centrifuges may be broken down by size, speed, or application. Size may be further differentiated by unit size, rotor capacity, or sample size. Speed (or g-force) refers to the centripetal force applied to the sample and varies significantly depending on the sample. At the very highest end are ultracentrifuges capable of separating molecules, cellular components, even isotopes. As rotational speed increases, samples and analytes tend to get smaller and separations more difficult.

Speed in rpms is the most common way to classify a centrifuge, although RCF (relative centrifugal force, or g) is more precise. Rpms are also the most common feature users ask for, according to Peter Will, product manager at Labnet International (a Corning Life Sciences company; Edison, NJ). “But g-force is the more critical number.”

Numerous rotor types are available for laboratory centrifugation. The two most common designs are fixed angle and swinging bucket. In the former, sample tubes remain in a fixed position on the rotor within x-y-z space; separations progress along the side of the tube and into a pellet at the bottom, depending on the actual angle.

In swinging bucket designs, as the centrifuge reaches terminal speed, samples swing out from a vertical position to where they become parallel to the floor or workbench; pellets collect neatly at the bottom of the tube.

Vendors strive to address as many centrifugation tasks and container options as possible within specific centrifuge designs. Hence the capability overlap among various models. “There is quite a range of options within different markets,” says Hugh Tansey, global commercial director for centrifugation at Thermo Fisher Scientific (Waltham, MA).

Care and maintenance

Centrifugation lore includes tales of violent malfunctions resulting in total destruction of the unit and much of the surrounding lab space. While these stories are unfortunately true, advanced centrifuge designs significantly reduce many potential safety issues through automatic rotor identification and inertia checks that shut the unit down if samples are improperly balanced. “Quality centrifuges will detect a mass imbalance quickly and both shut down and alert the user, usually through an audible or visual signal,” says Peter Will.

Bleach Towelettes for Centrifuges and Glucometers / HYPE-WIPE® and MINI HYPE-WIPE® / Current Technologies / www.currtechinc.comMuch more common than catastrophic failure are errors or breakdowns that lead to an unusable centrifuge or, perhaps as bad, lost samples. “Everyone in labs works so hard to get their samples to the point where they’re ready for spinning down,” Tansey says. “You don’t want to have them locked into the system or damaged or lost.”

Rotor acceleration and deceleration are critical for balancing the needs of sample preservation and rapid run time. Deceleration that is too rapid may cause disruption of the sample pellet, while rotor slowdown that is too deliberate can add considerably to run time. “Ideally, you want a rapid but soft stop,” Will says. “Quality centrifuges control this through electronics and programming that come standard with the device, where users can specify acceleration and deceleration rates.”

Another aspect of safe centrifugation is protecting operators from hazardous materials. Of concern are toxic chemicals and pathogenic bacteria and viruses. “Users working with these types of samples should look for models that offer rotors with aerosol-tight lids,” says Will.

Cleaning

Centrifuges are generally quite impervious to abuse. Still, manufacturers recommend regular cleaning, to the point of marketing their own detergents and specialized cleaning products.

Randall Lockner, marketing manager at Beckman Coulter Life Sciences (Indianapolis, IN), recommends wiping out the inside of the centrifuge “can” (main chamber) at least every few cycles or runs. This not only maintains an appearance of cleanliness but also prevents more serious fouling and cross-contamination. “Check all the lid and rotor O-rings for wear, cracking, and material buildup, and make this part of your maintenance routine.”

Many life science workflows still use radioactivity.

Depending on the isotope, centrifuges can become quite “hot” unless users check and/or clean the unit and rotors regularly. Equally serious is the potential for crosscontamination, which can skew results based on imaging, radiation scanning, or scintillation counting. Users should make swabbing with an appropriate solution that sequesters inorganic or organic materials, followed by counting, part of their routine whenever isotopes are used. Labs should take appropriate measures to protect workers from high-level spills and/or potentially dangerous swabs.

Most common maintenance problems involve rotors.

“Many life science applications employ salt buffers, and these can aerosolize or otherwise remain on the rotor after samples are removed,” says Matt Lieber, product manager at Eppendorf (Hauppauge, NY). Anodized aluminum and carbon fiber rotors are less susceptible to salt damage than conventional metal rotors, but these should still be cleaned regularly. Lieber suggests a non-charged detergent or a 70% ethanol solution applied with an absorbent paper towel.

Users should regularly check pivots on swing-out rotors for proper lubrication and apply silicone-based pivot grease regularly. Improper lubrication will not cause a disaster, but it may affect swing-out rate and skew results, particularly with phase separations (e.g., phenolchloroform extractions).

End-user maintenance

Regardless of their materials of construction, rotors need checking for wear and tear, which will degrade performance and potentially cause safety issues. Users should look for indications of wear, scratches, gouges, or effects of chemical exposure.

Regular maintenance visits should include inspection of all rotors to ensure that they are in service-worthy condition. Beckman-provided service personnel perform these inspections and leave behind a “report card” on rotor status. Keeping track of rotors near the end of their service lives can help lab managers prepare and budget for eventual replacement.

Centrifuge Tubes / ExtraGene / www.extragene-web.com“The simplest preventive step for extending the life of a centrifuge is to keep it well lubricated,” advises Jeff Antonucci, Northeast regional territory manager for Hettich Instruments (Beverly, MA). “Users should regularly check seals around the housing, as a ripped or broken seal can create a slew of problems. With refrigerated units, compromised seals can cause condensation and freezing within the chamber.”

Antonucci suggests that users “listen and feel” the centrifuge. “If you notice any vibration, shaking, grinding, or anything that doesn’t seem or sound right, stop the unit right away, inspect it, and if you can’t see the problem, call the manufacturer.”

Hazardous materials likely to be encountered during centrifugation include toxic chemicals, biohazards, and radioisotopes. No matter how careful the operator, spills, contamination, and cross-contamination are facts of life. Vendors are happy to advise users on preferred, noncorrosive cleanup techniques and products. Keep in mind, however, that what works for pathogenic viruses may provide insufficient decontamination for radioactive contamination spills.

Service

While vendors have relegated catastrophic centrifuge failures to the bad memory department, today’s users should be aware of the potential for bad results or loss of sample, which are almost always the result of substandard maintenance bred through adoption of poor centrifugation practices.

“All labs that depend on centrifuges should ensure that a professionally trained service engineer looks over the instrument at least once a year,” advises Randall Lockner. “Bottom line: Get a service agreement. Centrifuges are substantial investments, and if they go down, so does your laboratory.”

All service agreements should include some level of preventive maintenance by trained technicians. Beckman offers tiered service packages that may include additional coverage for mechanical repairs or replacements. Absent a service agreement, the cost of maintenance can add up, especially when parts, travel, and labor are factored in.

Centrifuge Rotors / FA-45-6-30 & A-2-DWP-AT Eppendorf / www.eppendorfna.comThe service engineer, whether an employee of the vendor, a third-party service organization, or an independent, is the primary resource for non-routine centrifuge care and maintenance. Top vendors will provide a technician at the time of acquisition to ensure that the instrument is installed and balanced properly and to provide training for users who need it. “The engineer will also determine whether the power supply and ventilation are appropriate for that unit and that users understand the basics of balancing samples, replacing rotors, and other basic functions,” Lockner notes.

Purchase decisions— What to consider

Centrifugation is a mature technology, but given its essential role in most laboratories, managers should take purchase decisions seriously. “Customers look for high-quality centrifuges and a strong brand reputation,” says Thermo Fisher’s Hugh Tansey. Users want results faster, so spin rate and rotor capacity should be at the top of a lab’s wish list.

Applications are the primary driver, says Randall Lockner, of which centrifuge category or model to acquire. “Are you separating nanomaterials? Are you doing mostly simple pelleting? Cell culture or cell fractionation? Consider what you need to retain and what you’ll be discarding in your samples and what feeds into that selection process.”

These considerations are where vendor interaction is most helpful, because a good deal of capability overlap exists among centrifuge models. “This becomes a challenge for new labs or new purchasers,” Lockner says. “It’s up to the vendor to explain differences among models based on the end user’s needs. For many labs, a general purpose tabletop model with the proper accessories will handle every conceivable workflow.”

Vendors foster this functional overlap by designing greater flexibility within each centrifuge product category. According to Tansey, rather than purchasing several systems or even a general-purpose floor model, labs can acquire one benchtop unit that’s easier to use and offers greater performance in a smaller footprint. These centrifuges sport intuitive displays and motorized, automated lids and latches. “We’ve replaced several platforms with future-ready centrifuge platforms with improved safety and rotor installation and greater usability,” Tansey says.

Labs that centrifuge large samples of three or four liters or more or many samples at once are probably destined for a floor-model centrifuge. Both tabletop and floor units are capable of sedimentation of nanoparticles and cell components, depending on throughput and capacity requirements. In fact, many labs have both types of centrifuge. “But expensive lab real estate leads to the drive to reduce not only the number of instruments but also instrument sizes,” Lockner says.

Managers should not discount lab environment in their purchase. Large centrifuges take up a lot of space and make a good deal of noise. Many organizations have begun to include centrifuges in core facilities, which are usually some distance (even several floors) away from where operators generally work. Luckily, the software capabilities of today’s centrifuges enable multiuser environments while maintaining traceability and safety. Some vendors now provide apps that allow users to monitor centrifuge operation remotely, for example when a run is completed or if the user ahead of you is not quite ready to give up the instrument.

Many labs still rely on older centrifuges that lack the capacity, capabilities, and user-friendliness of today’s units. They tend, says Tansey, to focus on today’s applications without regard to how projects and workflows might change. Modern designs can, through introduction of a new rotor or adapter, provide labs with great flexibility and performance. “Don’t be too grounded in what you think a typical centrifuge might bring you,” Tansey advises.

Purchasers should also focus on consumables. “Customers are not always cognizant of the tube type they work with,” says Lieber. While adapters exist for using major tube types with most microcentrifuge rotors, cryovials and HPLC tubes may require special adapters. “Tube types become even more challenging with large rotors, due to the wide variety of large tube formats.”

Lieber also suggests considering noise level, unit size, and physical profile. Users whose “office” consists of a few square feet of space on their workbench or at a nearby table may suffer hearing problems or become distracted by the noise from an often-used centrifuge. “Make sure you know where centrifuges will be located and how they will fit on the bench or floor.”

Tubes and rotors

Labnet’s Peter Will narrows down a customer’s centrifuge needs by first asking what size tubes they intend to spin, if they require a fixed-angle or swing-out rotor, the number of tubes they need to spin per run, whether they require refrigeration, and finally the g-force or rpms required to get the job done.

Centrifuge Tubes / Tube® 5.0 mL / Eppendorf / www.eppendorfna.com“Tubes per run is critical and a factor that I believe differentiates us from some of the competition,” Will says. Generally, the more tubes that fit within a given rotor footprint, the better, especially for larger, high-throughput labs. Analogously with speed/rpms, higher-capacity rotors accommodate fewer tubes, but smaller-capacity rotors are more limited.

Will agrees with the other experts interviewed for this article that ease of swapping out rotors is a critical attribute for centrifuges. Purchasers should be wary of units requiring more than about 30 seconds to switch rotors. Swapping difficulty and extended switch-out times are often indicative of poor ergonomics, heavier materials of construction, and sometimes potential sources of operating error. Intuitive control panels are a factor that readers of Lab Manager are familiar with for more operator-intensive instrumentation (e.g., HPLC, MS), but that applies just as aptly to centrifuges. Labnet and Hermle units, for example, have simple control panels that do not require cross-training as operators move from one instrument to another.

“Purchasers should not limit themselves to today’s workflows,” Will advises. “They should therefore select models that provide flexibility, both for current applications and potential future applications.” Flexibility relates to spinning speed as well as to the ability to accept rotors that hold larger or specialty tubes or even microplates.

Purchasers should also consider ease of use. “Customers don’t want to think too much about centrifuges,” Tansey says, “They just want them to work, especially in labs with high turnover or with multiple common users.”

Changing rotors, especially larger ones, used to be challenging for many lab workers. Large metal rotors used in floor-model centrifuges are particularly difficult to manipulate. “Historically this required the use of tools and physical strength,” Tansey says. Rotor changes were a significant source of damage to the unit, particularly when attempted by untrained operators.

Black Screw-Top Centrifuge Tubes / Asynt / www.asynt.comTop vendors have significantly reduced rotor change headaches by making rotors lighter and easier to engage and disengage from the centrifuge. Managers investing in new large centrifuges should therefore consider carbon fiber rotors, which are up to 60% lighter than equivalent metal rotors. Carbon fiber rotors are resistant to many chemicals that damage metal so may even last longer than their heavier counterparts. Thermo Fisher’s Auto-Lock system enables rotor changes through push-button control with no tools required.

Another consideration, particularly with swinging bucket rotors, is some sort of containment lid that protects users from materials that aerosolize from centrifuged samples. Biocontainment becomes the operative term with centrifugation of biohazardous cells and organisms, but in effect the lids will protect the lab from any hazardous material capable of volatizing.

As with rotors, installing biocontainment lids requires some skill and care. Thermo Fisher’s ClickSeal technology makes removing and applying the protective lids much easier, according to the company.

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